When I first started reading ReLIFE, the premise was so sketchy that I almost didn't continue. A medicine that can turn you young? Having the neet protagonist return to high school to relive (and subsequently fix) his life? I'm not the only one who thought this was either going to go the dark horror route or the harem route, am I?
But, nah, ReLIFE is legitimately good after you get past that initial, "Are you really using this type of setup?" It's a feel-good, slice-of-life, coming-of-age story about youth! And friendship! In other words: expect lots of heartwarm. And character development!
I have mixed feelings about the romance though (the romance for the main character), though the mangaka takes pains not to romanticize it (at least, not too much) or ignore the squick factor, which is nice. My only concern is that I'm worried about how it'll turn out, but the resolutions of the other conflicts were nicely done, so I'm not too worried.
Overall, if you're interested in a sweet, heartwarming coming-of-age story, ReLIFE slots nicely into that genre. If you're not interested, ReLIFE isn't so good that it'll change your mind either. What you see is kind of what you get.
edit: OK, so the mangaka has fixed the squick factor of the romance in probably what is the only way to fix it with zero squick factor left. Overall, I believe it's a good move in that sense, but as a narrative device, it ends up a bit unwieldy, even as the readers realize that the mangaka has been setting up this particular resolution for a while. I'm definitely still enjoying how further developments are unveiling from this revelation, though. And I'm continually impressed with the fact that the conflicts occur so organically, coming naturally from the person's personality, past actions, and interactions with other characters.
Definitely a worthwhile read if you want something with realistic interpersonal drama.